This invention relates to loaded paper and its production.
It is conventional to load paper with fillers in order, for example, to improve the opacity, whiteness and printability of the paper, and/or to reduce the cost of the paper (fillers are normally cheaper than the cellulose fibres which they replace). A drawback of the use of fillers is that the strength and other properties of the paper are impaired. This has had the effect of imposing limits on the proportion of filler which can be incorporated in the paper.
Fillers are normally incorporated in the paper web during its formation on the papermaking wire. This is achieved by having the filler present in suspension in the papermaking stock, so that as the stock is drained on the wire, suspended filler particles are retained in the resulting wet fibrous web. A problem with such a system is that quite a high proportion of filler is entrained in the water draining through the wire, rather than being retained in the web, and is therefore potentially lost. This problem is particularly serious with relatively lightweight papers. Although losses can be minimised to a considerable extent by re-use of this drained water in making up further papermaking stock, loss of filler as a result of imperfect retention in the web adds significantly to the cost of the paper produced.
As the cost of papermaking pulp, fillers and energy has increased, much effort has been devoted to the development of techniques which facilitate attainment of higher loading levels without unacceptable deterioration in paper properties, particularly strength and stiffness, and/or increased filler retention during formation of the web on the papermaking wire.
Such techniques have in the main involved the treatment of the filler particles and sometimes also the papermaking fibres, with one or more natural or synthetic polymers. These polymers may be charged in order to produce an interaction with the filler particles and/or the papermaking fibres, both of which are themselves normally negatively charged when in suspension in papermaking stock. A general review of the subject is to be found in a chapter entitled "Retention Chemistry" by J. E. Unbehend and K. W. Britt forming part of "Pulp and Paper-Chemistry and Chemical Technology", Third Edition, edited by James P. Casey, Volume 3, (Chapter 17). This Chapter discloses, inter alia, the sequential use of low-molecular weight cationic polymer followed by high-molecular weight anionic polymer, which is stated to offer particular benefits.
The patent literature also contains numerous proposals for filler treatment, and sometimes also fibre treatment as well. A number of these proposals are outlined below by way of example:
(i) UK Patent No. 1347071 discloses the treatment of fillers with cationic and anionic starches, so as to coat the filler particles with a coagulated or precipitated starch mixture. The coated filler is stated to exhibit improved retention characteristics. No pre-treatment of papermaking fibre with polymer(s) is disclosed.
(ii) UK Patent No. 1497280 discloses the treatment of filler particles with an anionic polymeric flocculant and a counter-acting anionic deflocculant. Papermaking fibres may be present during this treatment, and a cationic polymeric retention aid such as a polyacrylamide or a cationic starch may be added as a stock addition to the fibre/filler mixture. The treatment disclosed is stated to give improved strength at a given loading level, and hence to enable a higher proportion of relatively cheap filler to be included in a paper of given strength, which leads to considerable economic advantage. There is no disclosure of separate treatment of filler and papermaking fibre with polymeric materials, or of pre-treatment of filler with cationic polymeric material.
(iii) UK Patent No. 1505641 discloses the treatment of filler particles with an anionic latex, optionally after it has been treated with a cationic polymer such as a cationic starch. This treatment is stated to permit a high proportion of filler to be present in the paper without significant deterioration of mechanical properties. No pre-treatment of papermaking fibre with polymer(s) is disclosed.
(iv) UK Patent No. 1552243 discloses the treatment of filler particles with charged polymers, e.g. high molecular weight acrylamide polymers or copolymers, to form a filler/polymer conglomerate for use as a loading material in paper. Polymeric wet-or dry-strength resins may be present when the filler is treated. The treated filler is then mixed with papermaking fibre, after which polymeric retention aids may be added. A paper web is then formed in the normal way. The use of the treated filler is stated to permit increases in the filler content of the paper without substantially affecting the physical strength characteristics of the paper.
(v) UK Patent Application No. 2016498A discloses the treatment of filler particles simultaneously with inter alia, a cationic polyacrylamide and an anionic starch, and the use of the thus treated filler as a loading in paper. Excellent retention is stated to be obtained. There is no disclosure of treatment of papermaking fibres with polymer(s).
(vi) European Patent Application No. 50316A discloses the treatment of filler particles with a conventional papermaking organic binder and a cationic polymeric flocculant before being mixed with fibres. The fibres may be pre-treated with an anionic polymeric retention aid.
(vii) European Patent Application No. 60291A, equivalent to and published as International Patent Application No. WO/01020, discloses the reaction of a cationic starch with an anionic polyelectrolyte to form an "amphoteric mucus" which is then mixed with filler and/or papermaking fibres, after which an inorganic polymer of high surface charge is added to produce a partially dehydrated mucus gel-coated filler/fibre structure which is then used in a papermaking furnish. This is stated to give high filler retention and to produce papers of high strength and high filler content. Broadly similar proposals using different combinations of charged polymers are to be found in Swedish Patent Application Nos. 8201545A; 8201596A and 8205592A.
(viii) International Patent Application No. WO/02635 discloses the addition of a cationic starch of specified degree of substitution, an anionic polymer of specified molecular weight and a cationic synthetic polymer to a filler-containing papermaking stock in order to improve retention. There is no disclosure of the separate treatment of filler and fibre.
(ix) U.S. Pat. No. 4487657 (equivalent to European Patent Application No. 6390A) discloses the addition of an inorganic flocculant or an organic polymeric flocculant to an aqueous suspension of filler and fibres, followed by the addition of an organic binder, followed by a further flocculant addition. There is no disclosure of separate treatment of filler and fibre.
(x) European Patent Application No. 3481A discloses the addition of an aqueous mixture of filler and an ionically-stabilized charged latex to an aqueous fibre dispersion, followed by destabilization of the resulting mixture, for example by means of a charged polymer. A paper web is then formed in conventional manner. Normal papermaking additives may also be used.
(xi) UK Patent Application No. 2085492A discloses the addition of an ionic latex and at least one cationic polymer to an aqueous filler/fibre suspension which is then drained in conventional manner to produce a highly-loaded paper web suitable for use as a good quality fine printing paper. There is no disclosure of separate treatment of filler and fibre.
(xii) Japanese Laid-Open Patent Publication No. 55-163298 discloses pre-treatment of filler with a cationic polyacrylamide and pre-treatment of fibre with anionic polyacrylamide, after which the treated filler and fibre are mixed and a paper web is formed in conventional manner. The paper web is stated to have improved surface strength.
(xiii) German Offenlegungsschrift 3412535A discloses the addition of a polysaccharide, for example a cationic starch, and a synthetic retention aid to a papermaking pulp suspension. A pre-treated filler, for example a filler which has been anionically dispersed and then treated with cationic starch, may be added to the pulp suspension prior to formation of a paper web in conventional manner.
The patent literature also contains proposals for the treatment of papermaking fibres to improve paper strength. For example, U.S. Pat. Nos. 3660338; 3677888; 3790514; and 4002588 disclose treatment of papermaking fibres with "polysalt coacervates" derived by mixing dilute solutions of anionic and cationic polyelectrolytes. This is stated to give rise to paper of improved dry strength. European Patent Application No. 100370A discloses mixing an anionic polymer solution with a cationic polymer solution and then adding the resulting mixture to papermaking fibres. This is stated to give rise to a paper of excellent strength. European Patent Application No. 921A discloses the treatment of negatively-charged papermaking fibres with a mixture of a cationic latex and an anionic polymer and the use of the thus treated fibres for the production of a high strength paper composite. European Patent Application No. 96654A discloses the addition of an anionic sizing agent and a cationic retention aid to a pulp suspension which may also contain filler. Paper of good mechanical properties is stated to be obtained. UK Patent No. 1177512 discloses the treatment of papermaking fibres sequentially with a cationic component comprising both aluminium ions and a cationic polymer and an anionic component comprising an anionic polymer. This is stated to give a wet web having improved drainage characteristics. U.S. Pat. No. 3146157 discloses the use of polysulfonium and polycarboxylate resins for fibre treatment in order to obtain papers of improved strength. None of these patents disclosing fibre treatment to improve paper strength also discloses treatment of fillers with polymers.
An article entitled "The superfilled paper with rattle" by Lindstrom and Kolseth in "Paper", 5th Dec. 1983 discloses that paper of high filler content and high strength may be obtained by treating a filler/fibre mixture with both cationic starch and an anionic polyacrylamide or with other cationic polymer/anionic polymer combinations. A similar but somewhat longer article appears in STFI Kontakt, No. 3/82, at pages 3 to 5.
Other proposals for the treatment of fillers and/or fibres with natural or synthetic polymers to improve retention or paper strength and/or to obtain other effects may be found, for example, in UK Patent Specifications Nos. 11282551; 1353015; 1371600; 1429796; 1451108; 1527077; 1581548; 2001088A; 2009277A; 2016498A; and 2125838A; in U.S. Pat. Nos. 2943013 and 3184373; in European Patent Specifications Nos. 41056A; 80986A; and 132132A; and in International Patent Application No. WO 86/00100 (published after the priority date hereof).
A problem experienced with quite a number of the previous proposals is that while the processes appear promising at laboratory scale, or under carefully controlled larger-scale trial conditions, they fail to maintain their performance in regular production on the paper machine, where high shear forces are encountered. A further problem is that the polymers needed tend to be expensive, and so can only be used in small quantities which are perhaps inadequate to produce significant benefits. However, at least some of the technology disclosed in the publications reviewed above is though to have been commercialised, and this has enabled progress to be made with regard to the objectives stated earlier. Nevertheless, there is still scope for further progress, and this is the object of the present invention.